[WebAssembly] MC: Ensure that FUNCTION_OFFSET relocations are always against function symbols.

[sbc100]

Summary

The getAtom() method wasn't doing what we needed in all cases. We want the symbols
for the function which defines that section. We can compute this easily enough and we
know that we have at most one function in each section.

Once this lands I will revert rL331412 which is no longer needed.

Fixes PR37409

[sbc100]

• revert

[sbc100]

• revert

[yurydelendik]

looks good

[keith.walker.arm]

llvm/trunk/lib/MC/WasmObjectWriter.cpp

228

What is a fintining function?

[sbc100]

llvm/trunk/lib/MC/WasmObjectWriter.cpp

228

That is a great question! See rL332632

llvm/trunk/lib/MC/WasmObjectWriter.cpp

class WasmObjectWriter : public MCObjectWriter {
 
   // Stores output data (index, relocations, content offset) for custom
   // section.
   std::vector<WasmCustomSection> CustomSections;
   // Relocations for fixing up references in the custom sections.
   DenseMap<const MCSectionWasm *, std::vector<WasmRelocationEntry>>
       CustomSectionsRelocations;
 
+  // Map from section to fintining function.
+  DenseMap<const MCSection *, const MCSymbol *> SectionFunctions;
+
   DenseMap<WasmFunctionType, int32_t, WasmFunctionTypeDenseMapInfo>
       FunctionTypeIndices;
   SmallVector<WasmFunctionType, 4> FunctionTypes;
   SmallVector<WasmGlobal, 4> Globals;
   SmallVector<WasmDataSegment, 4> DataSegments;
   unsigned NumFunctionImports = 0;
   unsigned NumGlobalImports = 0;
   uint32_t SectionCount = 0;
void reset() override {
     WasmIndices.clear();
     TableIndices.clear();
     DataLocations.clear();
     CustomSectionsRelocations.clear();
     FunctionTypeIndices.clear();
     FunctionTypes.clear();
     Globals.clear();
     DataSegments.clear();
+    SectionFunctions.clear();
     MCObjectWriter::reset();
     NumFunctionImports = 0;
     NumGlobalImports = 0;
   }
 
   void writeHeader(const MCAssembler &Asm);
 
   void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,

 // Emit the Wasm header.
 void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
   writeBytes(StringRef(wasm::WasmMagic, sizeof(wasm::WasmMagic)));
   writeLE32(wasm::WasmVersion);
 }
 
 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                 const MCAsmLayout &Layout) {
+  // Build a map of sections to the function that defines them, for use
+  // in recordRelocation.
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    if (WS.isDefined() && WS.isFunction() && !WS.isVariable()) {
+      const auto &Sec = static_cast<const MCSectionWasm &>(S.getSection());
+      auto Pair = SectionFunctions.insert(std::make_pair(&Sec, &S));
+      if (!Pair.second)
+        report_fatal_error("section already has a defining function: " +
+                           Sec.getSectionName());
+    }
+  }
 }
 
 void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         const MCFragment *Fragment,
                                         const MCFixup &Fixup, MCValue Target,
                                         uint64_t &FixedValue) {
   MCAsmBackend &Backend = Asm.getBackend();
if (Type == wasm::R_WEBASSEMBLY_FUNCTION_OFFSET_I32 ||
       Type == wasm::R_WEBASSEMBLY_SECTION_OFFSET_I32) {
     if (!FixupSection.getKind().isMetadata())
       report_fatal_error("relocations for function or section offsets are "
                          "only supported in metadata sections");
 
     const MCSymbol *SectionSymbol = nullptr;
     const MCSection &SecA = SymA->getSection();
     if (SecA.getKind().isText())
-      SectionSymbol = SecA.begin()->getAtom();
+      SectionSymbol = SectionFunctions.find(&SecA)->second;
     else
       SectionSymbol = SecA.getBeginSymbol();
     if (!SectionSymbol)
       report_fatal_error("section symbol is required for relocation");
 
     C += Layout.getSymbolOffset(*SymA);
     SymA = cast<MCSymbolWasm>(SectionSymbol);
   }

llvm/trunk/test/MC/WebAssembly/debug-info.ll

 ; CHECK-NEXT:  }
 ; CHECK-NEXT:  Symbol {
 ; CHECK-NEXT:    Name: .debug_line
 ; CHECK-NEXT:    Type: SECTION (0x3)
 ; CHECK-NEXT:    Flags: 0x2
 ; CHECK-NEXT:  }
 ; CHECK-NEXT:]
 
+; generated from the following C code using: clang --target=wasm32 -g -O0 -S -emit-llvm test.c
+; extern int myextern;
+; void f2(void) { return; }
+;
+; int* foo = &myextern;
+; void (*ptr2)(void) = f2;
+
 target triple = "wasm32-unknown-unknown"
 
 source_filename = "test.c"
 
 @myextern = external global i32, align 4
 @foo = hidden global i32* @myextern, align 4, !dbg !0
 @ptr2 = hidden global void ()* @f2, align 4, !dbg !6
 
 ; Function Attrs: noinline nounwind optnone
 define hidden void @f2() #0 !dbg !17 {
 entry:
   ret void, !dbg !18
 }
 
 attributes #0 = { noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="generic" "unsafe-fp-math"="false" "use-soft-float"="false" }
 
 !llvm.dbg.cu = !{!2}
 !llvm.module.flags = !{!13, !14, !15}
 !llvm.ident = !{!16}
 
 !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
 !1 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !3, line: 4, type: !11, isLocal: false, isDefinition: true)
-!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 6.0.0 (trunk 315924) (llvm/trunk 315960)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 7.0.0 (trunk 332303) (llvm/trunk 332406)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5)
 !3 = !DIFile(filename: "test.c", directory: "/usr/local/google/home/sbc/dev/wasm/simple")
 !4 = !{}
 !5 = !{!0, !6}
 !6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
 !7 = distinct !DIGlobalVariable(name: "ptr2", scope: !2, file: !3, line: 5, type: !8, isLocal: false, isDefinition: true)
 !8 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !9, size: 32)
 !9 = !DISubroutineType(types: !10)
 !10 = !{null}
 !11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 32)
 !12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
 !13 = !{i32 2, !"Dwarf Version", i32 4}
 !14 = !{i32 2, !"Debug Info Version", i32 3}
 !15 = !{i32 1, !"wchar_size", i32 4}
-!16 = !{!"clang version 6.0.0 (trunk 315924) (llvm/trunk 315960)"}
+!16 = !{!"clang version 7.0.0 (trunk 332303) (llvm/trunk 332406)"}
 !17 = distinct !DISubprogram(name: "f2", scope: !3, file: !3, line: 2, type: !9, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !2, retainedNodes: !4)
-!18 = !DILocation(line: 2, column: 16, scope: !17)
+!18 = !DILocation(line: 2, column: 17, scope: !17)